advertisement Monolithic Synchronous Regulator Drives 4A Loads with Few External Components – Design Note 328 Joey M. Esteves Introduction The LTC ®3414 offers a compact and efficient voltage regulator solution for point of load conversion in electronic systems that require low output voltages (down to 0.8V) from a 2.5V to 5V power bus. Internal power MOSFET switches, with only 67mΩ on-resistance, allow the LTC3414 to deliver up to 4A of output current with efficiency as high as 94%. The LTC3414 saves space by operating with switching frequencies as high as 4MHz, enabling the use of tiny inductors and capacitors. The LTC3414 employs a constant frequency, currentmode architecture and can deliver up to 4A of output current. The switching frequency can be set between 300kHz and 4MHz by an external resistor; alternatively, it can be synchronized to an external clock where each switching cycle begins at the falling edge of the external clock signal. For improved thermal management, the LTC3414 is offered in a 20-lead TSSOP package with an exposed pad to facilitate heat transfer. The LTC3414 can be configured for either Burst Mode® operation, pulse-skipping or forced continuous operation. Burst Mode operation maximizes light-load efficiency and extends battery life by reducing gate charge losses at light loads—at no load, the LTC3414 consumes a mere 64μA of supply current. Forced continuous operation maintains a constant frequency throughout the entire load range, making it easier to filter the switching noise and reduce RF interference— important for EMI-sensitive applications. Pulse skip mode provides a good compromise between light load efficiency and output voltage ripple. The LTC3414 provides for external control of the burst clamp current level, in effect allowing the burst frequency to be varied. Lower Burst Mode operating frequencies result in improved light load efficiencies, but there is a trade-off between light load efficiency and output voltage ripple—as the Burst Mode frequency decreases, the output ripple increases slightly. In the LTC3414, the burst clamp is 01/04/328_conv adjusted by varying the DC voltage at the SYNC/MODE pin within a 0V to 1V range. The voltage level at this pin sets the minimum peak inductor current during each switching cycle in Burst Mode operation. Pulse skip mode is implemented by connecting the SYNC/MODE pin to zero volts. In pulse skip mode, the burst clamp is set to zero current and the minimum peak inductor current is determined by the minimum on-time of the control loop. Pulse skipping minimizes the output voltage ripple by providing the lowest possible inductor current ripple. High Efficiency 2.5V/4A Step-Down Regulator Figure 1 shows a 2.5V step-down DC/DC converter that is configured for Burst Mode operation. This circuit L, LT, LTC, LTM, Linear Technology, the Linear logo, Burst Mode and OPTI-LOOP are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 1 ROSC 294k 2 3 RSS 2.2M CSS 1000pF X7R 4 5 6 7 8 + CIN2*** 330μF 9 10 PGND PGND VFB RT ITH SYNC/MODE RUN/SS PGOOD SVIN SGND LTC3414 NC NC PVIN PVIN SW SW SW SW PGND PGND 20 19 18 C1 10pF X7R R3 124k CITH 470pF X7R R2 78.7k RITH 20k R1 432k 17 16 15 PGOOD RPG 100k CIN1 22μF X5R 2X VIN 2.7V TO 5V 14 13 12 11 L1* 0.47μH COUT2 22μF X5R VOUT 2.5V 4A + COUT1** 470μF DN328 F01 * VISHAY DALE IHLP-2525CZ-01 ** SANYO POSCAP 4TPD470M *** SANYO POSCAP 6TPB330M Figure 1. 2.5V/4A Regulator, Burst Mode Operation provides a regulated 2.5V output at up to 4A from a 2.7V to 4.2V input. Efficiency for this circuit, shown in Figure 2, is as high as 94% for a 3.3V input voltage. The switching frequency for this circuit is set at 1MHz by a single external resistor, ROSC. Operating at frequencies this high allows the use of a lower valued (and physically smaller) inductor. 1 ROSC 294k 2 3 RSS 2.2M 4 CSS 1000pF X7R In this particular application, Burst Mode operation maintains the high efficiency at light loads. The burst clamp current is set by the R2 and R3 voltage divider, which generates a 0.49V reference at the SYNC/MODE pin. This corresponds to approximately 1.2A minimum peak inductor current, as shown in Figure 2. 5 6 7 8 100 + TA = 25°C 95 CIN2*** 150μF 9 EFFICIENCY (%) 90 85 10 80 PGND PGND VFB RT ITH SYNC/MODE RUN/SS PGOOD SVIN SGND LTC3414 NC PVIN NC PVIN SW SW SW SW PGND PGND 20 CC 100pF X7R 19 18 R2 200k CITH 470pF X7R RITH 12.1k R1 634k 17 16 C1 22pF X7R PGOOD RPG 100k CIN1 22μF X5R 2x 15 VIN 5V 14 13 L1* 0.68μH 12 VOUT 3.3V 4A COUT** 100μF 2x 11 75 DN328 F03 70 * MURATA LQH66SNR68M03L ** TDK C4532X5ROJ107M *** SANYO POSCAP 6TPE150M 65 69 55 50 0.001 Figure 3. 3.3V/4A Regulator, Forced Continuous Mode 0.010 0.100 1.000 LOAD CURRENT (A) 10.000 DN328 F02 100 Figure 2. Efficiency vs Load Current 3.3V to 2.5V Burst Mode Operation from Figure 1 Data Sheet Download www.linear.com Linear Technology Corporation EFFICIENCY (%) High Efficiency 3.3V/4A Step-Down Regulator with All Ceramic Capacitors Figure 3 shows a 3.3V step-down DC/DC converter using all ceramic capacitors. This circuit provides a regulated 3.3V output at up to 4A from a 5V input voltage. Efficiency for this circuit, shown in Figure 4, is as high as 93%. Ceramic capacitors offer low cost and low ESR, but many switching regulators have difficulty operating with them because the extremely low ESR can lead to loop instability. The phase margin of the control loop can drop to inadequate levels without the aid of the zero that is normally generated from the higher ESR of tantalum capacitors. The LTC3414, however, includes OPTI-LOOP® compensation, which allows it to operate properly with ceramic input and output capacitors. The LTC3414 allows loop stability to be achieved over a wide range of loads and output capacitors with proper selection of the compensation components on the ITH pin. 90 80 70 60 50 40 30 20 10 0 0.01 0.1 1 LOAD CURRENT (A) 10 DN328 F04 Figure 4. Efficiency vs Load Current, 5V to 3.3V, Forced Continuous Conclusion The LTC3414 is a monolithic, synchronous step-down DC/DC converter that is well suited for applications requiring up to 4A of output current. Its high switching frequency and internal low RDS(ON) power switches make the LTC3414 an excellent choice for compact, high efficiency power supplies. For applications help, call (408) 432-1900 dn328f_conv LT/TP 0104 344K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2004